1. Field of the Invention
This invention relates to the field of hand-held probes for measuring the temperature of objects with which such probes are brought into contact.
2. Background Art
The assembly of delicate components and fine wiring used in modern miniaturized electronics requires soldering equipment which can operate within precise, narrow temperature limits. The need to verify that such equipment complies with its desired operational temperature ranges has spurred a search for temperature measuring devices which have an accuracy of +1.degree. C. in the range of 40.degree. C. to 500.degree. C. In hand-held probes, however, which normally use thermocouples for temperature measurement, several problems arise.
First, structural characteristics and inherent metallurgical impurities afflict any given thermocouple with a built-in measurement error margin which is a relatively constant quantity irrespective of changes in either the ambient or the measured temperatures. Thus the basic accuracy of a thermocouple between 100.degree. C. and 500.degree. C. is no better than +11.degree. C. This problem has been met to some extent by careful manufacture and selection of the wires that make up each thermocouple and by the use of calibration circuits which can be set to compensate electrically for the error margin built into the thermocouple with which the calibration circuit is operated.
In addition, however, the metals used in thermocouples are usually different from those used in the electronic circuitry of the instrumentation which processes the thermocouple temperature readings. For example, a common thermocouple, the Type J thermocouple, is made up of a junction between an iron and a constantan lead, while electronic circuits commonly have conductors made of copper. This means that where the output leads of the thermocouple are connected to an electronic circuit, a bimetallic junction will result. Such a junction itself tends to function in the manner of a thermocouple, adding or subtracting to the instrumentation system voltages which vary in amount as the temperature at those junctions changes.
This second source of thermocouple error is commonly dealt with in one of two ways. The output connections can be held at a constant temperature, as in an ice bath or in an oven of known temperature, converting the connections into reference junctions which produce an invariant thermoelectric effect that can be counteracted by additional circuitry. An ice bath or a constant temperature oven is too bulky to permit locating the reference junctions in the hand-held portion of a thermocouple probe. Yet the metals of which thermocouple output leads are commonly composed are too brittle to permit the leads to be extended long distances from the thermocouples before junctioning with the copper of an electronic circuit, as for example to the stationary console unit from which such probes are operated and in which units sufficient space does exist to place an ice bath or constant temperature oven.
In the alternative, yet another calibration element, one having a compensating electrical effect which varies with the temperature of the output junctions, is added to the circuitry. Careful design and selection of such a calibration element ideally permit its temperature dependent electrical effects to offset the variable errors in thermocouple readings caused in the output junctions by ambient temperature changes.
Measuring instruments which use thermocouples in hand-held probes generally require that the probes be interchangeable so that the combination of a new probe with the same instrumentation console need not be recalibrated when an old probe is replaced. This requirement further complicates calibration solutions to thermocouple errors, both those errors built in and those arising from the unwanted effects of output junctions. The problems in this regard are so numerous that only prohibitively expensive instruments using selected probes can meet the normal accuracy requirement range of +1.degree. C. over a normal range of ambient temperatures and a range of 100.degree. C. to 500.degree. C. in measured temperatures.
A third difficulty encountered with the accuracy of handheld thermocouple probes is that in measuring small items having high temperatures, the probe itself serves as a heat sink, drawing heat into itself and out of the measured item. This probeloading effect reduces the temperature of the measured item in the vicinity of the probe, resulting in a correspondingly inaccurate lower than actual temperature reading. A partial solution to this difficulty is found in making probe tips very fine, but such delicate equipment does not withstand everyday or repeated use.